Basophils are rare granulocytes that are most commonly found in peripheral blood and tissues. The primary function of basophils is to release histamine in response to allergens. Histamine causes blood vessels to dilate and increases blood flow to the inflicted area creating an inflammatory response. An increase in blood flow brings in a steady supply of white blood cells, such as eosinophils and neutrophils, to remove any foreign substance the immune system deems a threat. Basophils are also antigen-presenting cells (APCs) and promote the differentiation of naïve T cells into helper T cells.